1. Field of the Invention
The present invention relates to a recording head of an ink jet recording apparatus for performing recording by ejecting an ink from the recording head to a recording medium.
2. Related Background Art
Recording apparatuses such as a printer, a copying machine, a facsimile apparatus, and the like are designed to record an image consisting of a dot pattern on a recording medium such as a paper sheet, a plastic thin plate, or the like.
The recording apparatuses can be classified into an ink jet type, a wire dot type, a thermal type, a laser beam type, and the like. Of these apparatuses, an ink jet type apparatus (ink jet recording apparatus) performs recording by ejecting and flying ink (recording liquid) droplets from ejection orifices of a recording head, and attaching the droplets onto a recording medium.
In recent years, many recording apparatuses have been used, and they are increasingly demanded to meet high-speed recording, high-resolution, high-image quality, and low-noise requirements. As a recording apparatus which can meet these requirements, the ink jet recording apparatus is known. In this ink jet recording apparatus, since recording is performed by ejecting an ink from a recording head, a non-contact print operation can be realized, and hence, a very stable recorded image can be obtained.
However, since an ink jet recording system uses an ink as a fluid, various hydrodynamic problems occur when the system is used at a speed near or more than the upper-limit print speed of a recording head. Since an ink is a liquid, its physical states such as viscosity, surface tension, and the like considerably vary all the time depending on the environmental temperature or the leaving time of the ink. For this reason, even though a print operation is possible in a certain state, it may be disabled due to a change in environmental temperature, an increase in negative pressure caused by a decrease in ink remaining amount, or the like.
In many conventional apparatuses, in order to print a vertical line as linearly as possible, all of a plurality of nozzles are subjected to ink ejection within a time period as short as possible. For this purpose, several tens of nozzles are divided into groups each including several to 10 nozzles to shorten the ejection time as much as possible. In this case, when a recording head is used near an upper-limit ejection period, an ink cannot be refilled in the nozzles in time. For this reason, the next ejection is started before the ink is refilled, thus causing an ejection error and an extreme decrease in ejection amount. In particular, when ejection from a large number of nozzles is performed within a short period of time, the negative pressure level in a common liquid chamber temporarily becomes very high, and a refill operation cannot be performed in time. Also, a large vibration occurs due to resonance, and the next ejection is started in a state wherein the ink protrudes above the nozzle surface, resulting in splash-like ejection. In this manner, various vibration problems often occur in the conventional apparatus.
In order to solve the fluid vibration problems, some proposals have already been made. As one of these proposals, a method of forming a bubble in the common ink chamber or ink channels in a recording head is known. This method aims at solving the fluid vibration problems by the vibration absorption effect of the bubble. It is most effective to form a bubble near ejection nozzles which are basically vibration generation sources. As the bubble is formed nearer a tank, the effect is lessened, and a suppression effect of a high-frequency vibration generated by ejection is almost lost. Therefore, the bubble is formed in, e.g., the common ink chamber to obtain the vibration suppression effect.
However, the above-mentioned method suffers a problem of a long-lived bubble. Furthermore, since the method of absorbing a vibration using a bubble acts in a direction to absorb an ejection reactive pressure wave, it strongly acts as a vibration suppression effect. However, upon ejection of an ink, the backward impedance (an impedance at the upstream side of ink supply) of a foaming point also decreases, and conversion efficiency of foaming energy into ejection energy is impaired. Therefore, although the refill speed, the conversion efficiency, or the like can be increased or improved on the average, the original ejection characteristics of ejection nozzles cannot be perfectly utilized.
The setting position of an electrothermal energy converting element (to be simply referred to as a heater hereinafter) in the longitudinal direction of each nozzle will be discussed below.
As the heater position is selected to have a larger distance OH to an ejection orifice, energy efficiency becomes better. Foaming energy (ejection energy) obtained by driving the heater is consumed as energy for pushing an ink in both the direction of an ejection orifice and the direction of a common ink chamber. Typical factors influencing the impedance at that time include a fluid resistance (R) generated due to the viscosity of an ink upon movement of the ink, and an inertia (I) generated as energy for starting the ink to move. If the tube diameter of a nozzle is constant, since the fluid resistance (R) and the inertia (I) are proportional to the tube length, input energy can be efficiently converted into ejection energy by setting the OH for determining the forward impedance (an impedance at the downstream side of ink supply) to be sufficiently smaller than a distance HC to the common ink chamber, which distance determines the backward impedance (an impedance at the upstream side of the ink supply). However, the ink amount before the heater must be assured to have at least a capacity capable of forming a flying droplet, and it is impossible to infinitely decrease the OH. For this reason, the total length of the nozzle is prolonged to increase the backward impedance by setting the HC to be sufficiently large, so that the forward impedance becomes relatively small. With this arrangement, energy efficiency of input energy is improved.
However, in a conventional ink jet recording apparatus, the nozzle length is set to be relatively large to increase the backward impedance, and this arrangement is not suitable for a high-speed response. Refill power of an ink to a nozzle is mainly determined by the balance between the supply force of the ink generated by the surface tension (capillary force) of the ink itself, and the impedance of the nozzle upon refill of the ink. For this reason, a relative decrease in forward impedance by increasing the nozzle length behind the heater leads to an increase in impedance of the entire nozzle upon refill, and delays refill (supply) of the ink. Therefore, the above-mentioned technique is discordant with techniques for realizing high-speed operations of recent recording apparatuses.